Electrical System General


Special tools 900-1

Electrical system safety precautions 900-1

Voltage and polarity 900-2

Wiring, fuses and relays 900-3

Electrical test equipment 900-3


Wiring codes and abbreviations 900-3


Voltage and voltage drop 900-5

Voltage, measuring 900-6

Voltage drop, testing 900-6

Continuity, checking 900-7

Short circuits 900-7

Short circuit, testing with ohmmeter 900-7

Short circuit, testing with voltmeter 900-8


a. Wire Color Codes 900-4

b. Terminal and Circuit Numbers 900-4

Special Tools

1984 Porsche 911 Under Dash Wiring

Special tool

A Fluke 87 Automotive digital multimeter

B 1115 LED tester with thin spade probes

(source: Baum Tools Unlimited)

C T746 Wire stripper/crimper

(source: Baum Tools Unlimited)

D T748 One-step crimping pliers

(source: Baum Tools Unlimited)

Special tool

A Fluke 87 Automotive digital multimeter

B 1115 LED tester with thin spade probes

(source: Baum Tools Unlimited)

C T746 Wire stripper/crimper

(source: Baum Tools Unlimited)

D T748 One-step crimping pliers

(source: Baum Tools Unlimited)


A brief description of the principal parts of the electrical system is presented here. Also covered here are basic electrical system troubleshooting tips.

Special tools

^ Many procedures and tests in this partition of the manual require the use of some electronic test equipment. Read the procedure through before starting work.

Electrical system safety precautions

Please read the following warnings and cautions before doing any work on your electrical system.

• The Ignition system of the car operates at lethal voltages. People with pacemakers or weak hearts should not expose themselves to the ignition system. Extra caution must be taken when working on the ignition system or when servicing the engine while it is running or the key is on. See 280 Ignition System for additional ignition system warnings and cautions.

• Before operating the starter without starting the engine (as when making a compression test), disable the ignition system by removing harness connector from the DME main/fuel pump relay. See 240 Fuel Injection.

• Always turn off the engine and disconnect the negative

(-) cable from the battery before removing any electrical components.

• Connect and disconnect ignition system wires, multiple connectors, and ignition test equipment leads only while the ignition is off.

• Disconnecting the battery cables erases engine control module (ECM) adaptive memory. It may be necessary to drive the car for approximately 10 minutes after reconnect!-ung the battery to reset ECM adaptive memory and restore normal engine performance.

• Do not disconnect the battery with engine running.

• Do not quick-charge the battery (for boost starting) for longer than one minute, and do not exceed 16.5 volts at the battery with the boosting cables attached. Wait at least one minute before boosting the battery a second time.

• Do not use a test lamp that has a normal incandescent bulb to test circuits containing electronic components. The high electrical consumption of these test lamps may damage the components.

' Do not use an analog meter. Use only a digital multimeter.

• Solid-state modules are static sensitive. Static discharge will permanently damage them. Always handle the modules using proper static prevention equipment and techniques.

• To avoid damaging harness connectors or relay panel sockets, use jumper wires with flat-blade connectors that are the same size as the connector or relay terminals.

• Always switch a digital multimeter to the appropriate function and range before making test connections.

• Do not try to start the engine of a car which has been heated above 176°F (80°C), (for example, in a paint drying booth). Allow it to cool to normal temperature.

• Disconnect the battery before dang any electric welding on the car.

• Do not wash the engine while it is running or any time the ignition is switched on.

Voltage and polarity

The vehicle electrical system is a 12-volt direct current (DC) negative-ground system. A voltage regulator controls system voltage at 12.6 -14.5 volts. All circuits are grounded by direct or indirect connection to the negative (-) terminal of the battery. A number of ground connections throughout the car connect the wiring harness to chassis ground. These circuits are completed by the battery cable or ground strap between the body and the battery negative (-) terminal.

Wiring, fuses and relays

Nearly all parts of the wiring harness connect to components of the electrical system with keyed, push-on connectors that lock into place. Notable exceptions are the heavy battery cables and the starter wiring. The wiring is color-coded for circuit identification.

With the exception of the battery charging system, most electrical power is routed from the ignition switch or the battery through the main fuse/relay panel, located in the left rear corner of the luggage compartment. Fuses are color coded to indicate current capacities.

The relays and control units/modules are mounted in various places throughout the vehicle. See 971 Electrical Component Locations.

Electrical test equipment

Many of the electrical tests described in this manual call for measuring voltage, current or resistance using a digital multimeter (DMM). Digital meters are preferred for precise measurements and for electronics work because they are generally more accurate than analog meters. The numerical display is also less likely to be misread, since there is no needle position to be misinterpreted by reading at an angle.

An LED test light is a safe, inexpensive tool that can be used to perform many simple electrical tests that would otherwise require a digital multimeter. The LED indicates when voltage is present between any two test-points in a circuit.

• Use a digital multimeter with at least 10 megaohm input impedance, or an LED test light. An analog meter (swing-needle) or a test light with a normal incandescent bulb may draw enough current to damage sensitive electronic components.

• An ohmmeter must not be used to measure resistance on solid state components such as control units or time delay relays.

• Disconnect battery before making resistance (ohm) measurements on the circuit.


The wiring diagrams shown in 970 Electrical Wiring Diagrams have been specially designed to enable quick and efficient diagnosis and troubleshooting of electrical malfunctions.

Wiring codes and abbreviations

A lot of information is included in each wiring diagram if you know how to read them. Wire colors in the diagrams are abbreviated. Combined color codes indicate a multi-colored wire. For example the code BLU/RED indicates a blue wire with a red stripe. See Table a.

Many electrical components, connectors, fuses, and ground locations are identified using a unique number. Each of these numbers correspond to a particular part in the circuit commonly found in 970 Electrical Wiring Diagrams.

Sometimes the color of an installed wire may be different than the one on the wiring diagram. Do not be concerned. Just be sure to confirm that the wire connects to the proper terminals.

Table a. Wire Color Codes





















Most terminals are identified by numbers on the components and harness connectors. The terminal numbers for major electrical connections are shown in the diagrams. Though many terminal numbers appear only once, several other numbers appear in numerous places throughout the electrical system and identify certain types of circuits. Some of the most common circuit numbers are listed below in Table b.

Table b. Terminal and Circuit Numbers


Circuit description


Low voltage switched terminal of coil


High voltage center terminal of coil

X or +X

Originates at ignition switch. Supplies power when the ignition switch is in the PARK, RUN or START position


Originates at ignition switch. Supplies power when ignition switch is in RUN or START position


Battery positive (+) voltage. Supplies power whenever battery is connected. (Not dependent on ignition switch position, unfused)


Ground, battery negative (-) terminal


Supplies power from battery to starter solenoid when ignition switch is in START position only


Originates at ignition switch. Supplies power when ignition switch is in the RUN position only


Ground side (-) of relay coil


Power-in side (+) of relay coil


Relay actuated contact


Alternator warning light and field energizing circuit

Additional abbreviations shown in the wiring diagrams are given below.


air conditioning

. digital motor electronics electronic control module

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